The invention relates to a method for preparing a biodegradable filter material from renewable raw materials for the use as tobacco smoke filter elements of cigarettes, cigars or pines.
Smoker articles such as for example cigarettes have a cylindrical shape in which the shredded smokable tobacco material is surrounded by a paper wrap. The majority of said cigarettes have at the one end a filter which is connected to the cigarette by means of a band. Filter elements and cigarette filters are extensively described in the literature as filter tows. For the preparation of cigarette filters usually a fiber material made of the materials cellulose-2,5-acetate or polypropylene is used. The use of paper or cotton wool is known either. According to a known method a cellulose acetate fiber material is prepared in most cases by the nozzle (spinneret) spinning process. From the cellulose acetate filaments and/or cellulose acetate spun fibers which are curled or crushed in a compression chamber, the filter tows are at first prepared as filter rods by stretching the curled ribbon, increasing it in volume and bringing it in the desired dimension in a formatting device and wrapping it with paper. The cellulose-2,5-acetate raw materials are normally compounded with the softener clycerin acetate which is contained in the tobacco smoke and may cause problems. With respect to the definition and description of a filter tow and tobacco filter element it is referred to DE-A-41 09 603 and DE-A-10 79 521. Methods for the preparation of filter tows and filter cigarettes are explained i.a. in the documents U.S. Pat. No. 5,402,802, DE-A-41 09 603, JP-A-5-377 812, EP-A-0 285 811, WO 93/02070, JP-A-5-392 586, WO 92/15209 and EP-A-0 641 525. Moreover, a plurality of suggestions for the preparation and use of biodegradable cigarette filters, which are prepared on the basis of cellulose ester and/or polyhydroxy butyric acid (PHB) or a copolymer of polyhydroxy butyric acid/polyhydroxy valeric acid (PHB/PHV), have been published, e.g. DE-A-43 22 965, DE-A-43 22 966, DE-A-43 22 967. Complex solutions are know for the problem of achieving an accelerated biodegradability of cellulose diacetates, which under normal climate conditions degrade in one to two years only (M. Korn: "Nachwachsende und bioabbaubare Materialien im Verpackungsbereich" [Renewable and Biodegradable Materials in the Packaging Sector], first edition, 1993, publishing house Roman Kovar, Munich, page 122). EP-A-0 632 968 suggests the use of enzymes which split cellulose chains, and DE-A-43 22 966 suggests the use of the degradation-increasing additives urea and urea derivatives. Also EP-A-0 632 970 is based on the problem of accelerating the degradation rate of cellulose acetate filters, which is to be solved by adding nitrogen compounds. DE-A-43 25 352 suggests to use a cellulose acetate which is modified with .epsilon.-caprolactone for the preparation of filaments. EP-A-0 632 969 shows a degradable cellulose acetate with a low substituation coefficient (a cellulose acetate with a substituation coefficient of &gt;2 is regarded as hardly degradable). EP-A-0 597 478 discloses a cellulose acetate with a substituation coefficient of &lt;2.15 and degradation-accelerating additives such as polycaprolactone. EP-A-0 634 113 describes a tobacco filter and a method for its preparation on the basis of cellulose ester monofilaments by the use of up to 30% water-soluble polymers, e.g. starches, in order to improve the degradability of the filter tow. In order to improve the degradability of cigarette filters on the basis of cellulose acetate (fibers), EP-A-0 641 525 suggests the co-application of wood pulp. Also U.S. Pat. No. 5,396,909 describes a cigarette filter with a filter tow from cellulose acetate. WO 93/07771 describes a method for the preparation of a cigarette filter from cellulose-2,5-acetate, the degradation rate of which should be accelerated by the co-application of starch. EP-A-0 597 478 relates to a biodegradable cellulose acetate with a substitution coefficient of 1.0 to 2.15 for the use as a raw material for the preparation of i.a. cigarette filters. EP-A-0 539 191 shows a low-weight cigarette filter in which the filter material partly consists of a closed-pore foam. Thus, a reduction of the filter weight is achieved. An improved biodegradability is disclosed in DE-A-40 13 293 and DE-A-40 13 304 which is achieved by using the biopolymer polyhydroxy butyric acid and/or the copolymer polyhydroxy butyric acid/polyhydroxy valeric acid (PHB/PHV) as the fiber raw material for the preparation of a filter tow.
EP-A-0 614 620 describes a biodegradable filter element in the form of a foam or a film on the basis of starch. The filter material is prepared by extrusion. The extruder arrangement comprises a plurality of temperature zones.
GB-A-2 205 102 describes a method for preparing cigarette filters from an extruded polysaccharide material such as, e.g., starch, in the form of a film, foam or strand. Biodegradable starch fibers and their use in cigarette filters are also known from EP-A-0 541 050.
As can be seen by this variety of solutions, based on the increased environmental consciousness there is the need for an improved filter material, e.g. for cigarette filters, having good biodegradability properties.